The scanning electron microscope is an apparatus for accelerating the primary electron discharged from the electron source, converging the same by an objective lens to narrow down the primary electron beam, scanning the sample with the primary electron beams by using a scanning deflector, detecting secondary signals generated from the sample by the irradiation of the primary electron beam and displaying this detected signal intensity as observation image. In order to obtain highly contrasted observation images, an efficient detection of secondary signals is required. In order to make a detector arranged outside the optical axis detect efficiently secondary signals, it is necessary to apply an electromagnetic field on the optical axis to deflect the secondary signals. However, this electromagnetic field increases the aberration of the primary electron beam. Therefore, in order to obtain a high-definition observation image enlarged at a high magnification, it is necessary to reduce the aberration of the primary electron beam. Therefore, it is necessary to contain the aberration of the primary electron beam generated on the optical axis between the electron source and the focus point on the sample.
JP-A No. 7(1995)-192679 discloses the technology of separating the orbit of the secondary electrons and the reflected electrons by using a deflector for deflecting the secondary electrons out of the axis of the primary electron beam and detecting selectively the secondary electrons and the reflected electrons.
JP-A No. 9(1997)-171791 shows an example of reducing the aberration amount of the primary electron beam generated in the secondary electron deflection field to detect efficiently the secondary electrons. JP-A No. 9(1997)-171791 discloses the technology of providing a secondary electron conversing electrode in which an opening for allowing the passage of the primary electron beam above the secondary electron detector, converting the secondary electrons generated or the reflected electrons from the sample that had collided with the secondary electron conversing electrode into secondary electrons, deflecting and detecting the same by the detector by using the secondary electron deflector to which the electromagnetic field is applied orthogonally to the secondary electron suction electric field, and canceling deflection by the electric field and deflection by the magnetic field by orthogonalizing the suction electric field and the magnetic field and correct the same. This technology is a method of detecting indirectly the secondary electrons (indirect detection method of secondary electron), and the deflection amount of the secondary electron can be small, and the impacts on the primary electron beam will be small in comparison with the direct detection method of deflecting the secondary electron substantially towards the detector.
However, in the configuration of the detector disclosed in JP-A No. 9(1997)-171791, the energy width of the primary electron beam generates deflection chromatic aberration in the secondary electron deflection field, and expands the diameter of the primary electron beam by several nanometers. For a scanning electron microscope in which a nm-order resolving power is required, the deflection chromatic aberration described above is not negligible.
On the other hand, a technology for reducing the deflection chromatic aberration by the secondary beam deflector is disclosed in JP-A No. 2001-357808. JP-A No. 2001-357808 discloses the technology of canceling the deflection aberration of the primary electron beam generated in the deflector by installing a secondary electron deflector that applies magnetic field orthogonally to the secondary electron deflection magnetic field in order to deflect the secondary electrons, and by installing another deflector to be operated under the condition that the polarity of deflection direction will be directly opposite to the electron source side from the deflector mentioned above.